1. Field of the Invention
In general, the present invention relates to systems and methods used to charge a rechargeable battery. More particularly, the present invention relates to recharging systems that actively monitor specified variables during a recharging cycle.
2. Prior Art Description
Rechargeable batteries come in many shapes and sizes. Small rechargeable batteries are used to power cell phones, toys, cordless tools and a large variety of everyday items. Large rechargeable batteries are used in cars, trucks, boats and the like. Even larger racks of rechargeable batteries are used to store power in electric vehicles, hybrid vehicles, commercial airliners, and many buildings that collect power using solar panels.
In order to charge a rechargeable battery, a current must be passed into the battery at a voltage that is greater than the output voltage rating of the battery. For example, in order to recharge a 12 volt battery, current must be supplied to the battery in excess of 12 volts. This voltage differential is required to ensure that electricity is flowing into the battery and not out of the battery. However, determining the best charging voltage for a particular battery or series of batteries is a complicated endeavor.
If a battery is overcharged, it can damage the battery and it can even catch fire. Battery fires due to overcharging have been reported in laptop computers, electric vehicles, and commercial aircraft, even though these applications use sophisticated systems to monitor the status of the rechargeable batteries. The reason for the batteries being overcharged is that prior art recharging systems are designed assuming that the battery and the wiring that leads to the battery have a low and constant resistance. In reality, this assumption is not true. Rechargeable batteries are not static systems. Rather, they are dynamic systems that vary in resistance in response to a variety of variables. For instance, the resistance of a rechargeable battery and its surrounding wiring is dependent upon the temperature of the battery, the age of the battery, the condition of the chemicals within the battery, port corrosion, and internal electrode degradation.
In the prior art, a conditioned DC power source is typically used to recharge a battery. The current is conditioned to a preselected charging voltage and charging current that is fed into the rechargeable battery. The output of the battery charger is monitored using a voltage comparator. The voltage comparator compares the output voltage of the battery charger to a preselected voltage set point. For example, a rechargeable battery may be nominally rated with an output voltage of 12.2 volts. The voltage set point for the comparator may be set at 13.8 volts. Thus, when the output voltage of the battery reaches 13.8 volts, the battery is deemed fully charged and the recharging voltage is stopped.
This prior art recharging methodology, assumes that the output voltage being received at the voltage comparator is exactly equal to the voltage output actually being produced by the rechargeable battery. This is a false assumption that creates significant problems. The wires that lead to and from the rechargeable battery have internal resistances. The battery itself has an internal resistance. The termination between the wires and the battery may be partially corroded and can provide significant resistance. In accordance with Ohm's law, when a current passes through a resistive element, a voltage is developed that varies as a function of the current times the value of the resistance. Consequently, the resistances of the wires, battery and contacts inflate the output voltage of the battery. The inflated voltage is read by the voltage comparator. The voltage comparator, therefore, believes that the battery is outputting more voltage than it actually is. The voltage comparator will, therefore, turn off the recharging current before the battery has become fully charged.
The difference between the inflated voltage and the real voltage of the battery is the error voltage. The recharging system is designed with a voltage set point at the comparator that can be compensated manually for a theoretical error voltage. However, the error voltage varies with time, temperature and battery degradation. Thus, the recharging system may overcharge the battery while attempting not to undercharge the battery. This is what often causes batteries to become overcharged and become damaged and perhaps catch fire.
A need therefore exists for a charging system for a rechargeable battery that dynamically monitors the voltage error inherent in the system. By adjusting the charging system as a function of the voltage error, the rechargeable battery can always be fully charged without the danger of overcharging the battery. This need is met by the present invention as described and claimed below.